I am writing a novel of historical fiction about the Second World War. It is written in the POV of several German and Russian characters but is intended for English readers. Which units of measurement should I use when referencing distance, weights and time? Miles or kilometers, pounds or kilograms, 24-hour or 12-hour time designations?
It's not "history" but rather "geography" that determines whether you should use kilometers or miles.
If your characters are German or Russian, use kilometers, because that's what they use, even today. (Depending on the time, e.g. Tolstoy in the 19th century, a Russian might use "versts", which are just a bit more than a kilometer.)
On the other hand, if you were writing an American Civil War novel, use miles, because that's what nineteenth century Americans used.
If you're looking at this from the perspective of a set of characters talking to each other, it would depend on their background. If their country used metric, then they would have metric as their instinct. If you want them to 'convert' because of the presence of an American character, for example, you can do what we Canadians sometimes end up doing which is "It's about half a meter... hang on, two and a half centimeters per inch, it's about 20 inches." From that point you could have the "metric" character providing US Customary units or Imperial, or whatever (or the reverse).
At the end of the day, you don't want to break your reader out of the story. You don't want them to feel like you're ignorant of what was used, and you don't want to make things awkward. Doing something like this, or finding another means, allows you to have the transition to "what you assume your reader is most comfortable with." However, whenever they would come across "official documents" or they would receive orders, you could keep it in the "other" system, which would also allow you to create a sense of distance and formality about those issuing the orders.
I hope that helps.
I would just use kilometres, it's a fairly world-wide unit of measurement, known even in the English speaking world. Most people have a vague idea of how far it is even if they can't easily convert from kilometres to miles in their head - I imagine even more so if they are interested in military fiction.
Besides, don't (Didn't) even the American military use the slang "klicks" to mean kilometres? You'd often see it used in American media by American characters for an American audience - and they don't seem too bothered about the audience knowing exactly how many miles that is (not to mention what the hell a "klick" is)
Not sure if they did as far back as WW II but your audience are modern English speakers.
If exact distance is a factor, you could always add in extra information like ("so it will take fifteen minutes to get there").
The Metrification of the U.S. Military can be dated as far back as the 1918, when Klick became slang for Kilometer was introduced. Guns from 1957 onwards are in metric (legacy still use imperial measurments). Aircraft use pounds for ordinance, but most military vehicles are built to metric standards. U.S. Navy still uses inches for guns, and the submarine fleet still uses feet for depth and velocity and the "Kiloyard" for distance. Other wise the Navy and airforce use Nautical Miles for distance and Knots for speed, both of which are metric unites of measure. Civilian and NATO military aviation all collectively use feet for height.
With this in mind, the average U.S. reader will more than likely accept metric units as the accepted use in military jargon, even from their own troops.
I am not versed in who used what when, so I can't tell you exactly what to use. Fortunately, I can tell you how to find out.
In most fiction, your write for your readers. So if your audience is German or Russian, you use the Metric System. If they are American, you use the US Customary System
In historical fiction, Germans or Russians would use the metric system, so they should do so within your novel. You can always include a conversion somewhere within your novel if the detail is important, or even just a vague description of the distance: "A field away," "A castle the size of a pigeon on the horizon," "a stone's throw."
However, when you the author are talking to the reader (ie, narration, no character is speaking), you should use the Us Customary Units. It might seem a bit jarring, so if possible, you might want to not use exact measurement units all the time. Rather, use descriptions like I included above, or simply things like, "arm's reach" and "a stride away" for short distances.
I hope that answers your question, and good luck in your writing!
I would say: In general, use the system familiar to your readers.
German officers during World War 2 presumably spoke German. But if I wrote an historical novel intended for American readers, I would give the dialog in English, for the obvious reason that most of my readers do not understand German. The reader understands that the dialog is intended to be a translation.
Likewise, I'd generally give units of measure in the Imperial system, because that is what the reader is familiar with.
I'd say this is borderline because most Americans are at least generally familiar with the metric system, so they wouldn't likely have serious trouble understanding the units that German officers would really have used.
There can be an issue if the actual numbers matter. For example, if I was reading a novel where a German character said, "Our reinforcements are still 13 miles away. 13 is proving a very unlucky number for us today", as a reader I might balk and say, "But wait, he would have really said 21 kilometers. Why would the character relate 21 kilometers to the unlucky number 13? This doesn't make sense." (Assuming Germans think of 13 as an unlucky number. Just making an example.)
I'd avoid using one set of units in dialog and another set in narration. I think that would get very confusing. Readers would have to convert back and forth to make sense of the story.